This invention relates to a resistant welding system and more particularly to a welding control system for eliminating magnetizing current transients in the primary of a welding transformer by reducing the residual currents in the secondary to zero at the end of each welding pulse in a multiplulse welding sequence.
In the current state of the art, the use of smaller welding transformers with sharply saturating iron cores has made the control of magnetizing current a matter of critical importance. The large capacity supply circuits and heavy conductors used in resistance welding allow extremely large magnetizing currents to flow when the transformer iron becomes saturated. This problem has long been solved for the "turn on" case where the first half cycle of the weld is carefully controlled to prevent saturation during the weld. At the end of the weld the welding electrodes (tips) are opened and the residual secondary current terminates. In cases where the tips do not open before the next welding pulse starts, the residual current biases the transformer iron sufficiently to cause a high magnetizing current to be drawn upon initiation of the next welding pulse. The high magnetizing current results in shorter transformer life, less current delivered to the workpiece and, voltage dips in the supply bus with resulting deleterious effects on other welders on the same supply bus which have welds in process.